The present invention concerns a handpiece for dental or surgical use, of the type comprising: a hollow shaft rotating about a longitudinal axis and mounted via roller bearings in a fixed tubular shaft; a collet mounted in a front end of the hollow shaft and having a central channel for receiving the shank of a removable tool, the collet including axial arms distributed around the axis and each provided with a clamping or gripping jaw in the central channel; a gripping mechanism carried by the hollow shaft and arranged to exert a centripetal force on a support surface for each arm of the collet to grip the shank of the tool between the jaws of the collet; and a releasing mechanism carried at least in part by the fixed shaft and arranged to act on the gripping mechanism at least so as to release the collet.
In usual handpieces, like those disclosed in FR Patent Application No. 2 191 869 corresponding to U.S. Pat. No. 3,902,248, the jaws are located at one end of the collet which has external support surfaces of conical shape, which engage in an inner cone of a tubular part which thus grips the end of the collet on the shank of the tool. For example, the collet is slidably mounted in the hollow shaft, it jaws and its external support surfaces are at its front end for cooperating with the front orifice of the shaft, and a collet spring keeps the collet gripped by pulling it axially backwards. The releasing mechanism comprises an outer sleeve which is connected to the body of the instrument by a screw system and which acts as a control member. When the user pivots the sleeve, the latter moves forward driving an inner bush which compresses the collet spring and moves the collet forward to release it from the cone that was gripping it.
In cases where the tool carried by the handpiece has to support relatively high stress, particularly as far as torque and axial force is concerned, the collet has to grip quite hard. This means the collet spring has to have quite high axial force, and high friction forces on the conical surfaces which transform the axial force into radial clamping or gripping forces. The releasing mechanism has to be able to overcome these forces by applying sufficient axial thrust to the sliding member biased by the collet spring. This axial thrust is then transmitted to the frame of the handpiece via at least one of the roller bearings supporting the rotating shaft. Most of the time, like in the construction according to FR 2 191 869, this thrust is exerted forwards and thus loads the roller bearings in the opposite direction to the axial loads that the tool will undergo in use, i.e. the roller bearings have to be designed to withstand axial loads in both directions.
Moreover, a relatively strong collet spring has a relatively large mass, which constitutes a considerable drawback for a part that rotates very fast and which cannot generally be centred with precision. The resulting vibrations increase with the mass of the spring.
In the handpiece disclosed in U.S. Pat. No. 3,631,697, the collet and its collet spring device are of the same type as that described hereinbefore, with the same drawbacks. In order to overcome the force of the collet spring, a releasing mechanism is provided comprising balls engaged both in helical slots of a fixed tube and in longitudinal grooves of a pivoting sleeve that acts as a manual control.
A handpiece without any collet spring is disclosed in U.S. Pat. No. 3,488,850. The collet has conical external surfaces cooperating with annular parts which produce its gripping and it is mounted in the hollow shaft with longitudinal play. The front end of the collet is not supported by the shaft, but by a front bearing fixed in an axially mobile nose to act as manual control member. In order to do this, the nose is mounted via a threading onto the body of the handpiece, such that the operator can rotate it to move the front bearing backwards or forwards in order to tighten or release the collet. A big drawback of this system is that it imposes a strong axial load on the bearings while the collet is being clamped, and thus during work. Moreover, the front bearing support system is not rigid enough to prevent the tool vibrating as soon as speed is high.